Also, some other errors: Grenade hopping can be done forwards as well as backwards, and will get you more air, but at the cost of you running over the explosion and taking damage for it. You also do not *need* to grenade hop anywhere in the game except to get to secret areas, so it's not that important a skill to learn. If it was that important that you could jump, there would be a jump key. You can also grenade CLIMB in Marathon 1, though that's not relevant except in the actual Mac version, not M1A1. Grenades, as well as rockets and fire, will make aliens not leave their weapons or ammo. Any exposive or fire, basically. This includes Bobs in M2/Infinity, if they get torched by aliens or you and you want their stuff. For propping doors open, the timing doesn't have to be right BEFORE it finishes opening. Most doors will pause while open before closing again; if you deactivate the door (presuming it is switch-driven) at any point in it's cycle, including sitting there paused, it will stay where you leave it. Otherwise, nice little tutorial.